This part provides background information related to the present disclosure, and it is not necessary to be the prior art.
The D2D (Device-to-Device) communication technology refers to an information transmission way, in which, a cellular communication UE (User Equipment) performs data interaction directly in a way of terminal direct connection. Compared with traditional cellular communication, in the D2D communication, a spectrum resource is reused, a transmission distance is short and information is not relayed through a base station, therefore the D2D communication can increase a spectrum utilization ratio, decrease a transmission power of the UE and load of the base station. In a case that a DUE (Device-to-Device User Equipment) which is performing the D2D communication is to switch from a D2D communication mode to a traditional cellular communication mode, design for a switch flow and an involved signaling is a part not involved in current standard work. However, since that the switch from the D2D communication mode to the traditional cellular communication mode is different from a traditional switch mechanism of the UE from a base station to another base station, and reflects that a communication target of the UE is switched from a UE to a base station, that is, from a nontraditional communication mode to a traditional communication mode, the switch flow in this mechanism needs to be planed and designed according a particular scenario, thereby ensuring rationality and reliability of the switch flow while reducing signaling overhead to the greatest extend.
In addition, D2D broadcasting is an important concern of the current 3GPP (3rd Generation Partnership Project) RAN1 (Radio Access Network), and is mainly applied to the field of public safety. The D2D broadcasting is point to multiple points communication, a source DUE is a broadcasting source UE for providing a broadcasting service, and the source DUE provides same information to the remaining DUEs, and does not receive feedback information from the UE. Due to a particular factor, the D2D communication is switched to other mode from the broadcasting mode in a case that quality of a D2D broadcasting link gets worse and can not meet a service requirement of a receiving DUE, to ensure completeness of receiving the information. However, a specific flow and scheme for switching the D2D communication from the broadcasting mode to the other mode is not discussed in the conventional technology, for example, which entity and how to trigger the mode switch without the feedback information. In some known study, a UE cluster is constructed in a case that multiple DUEs which meet a certain distance condition have a communication requirement. An advantage of the D2D communication cluster is centralized control, that is, the base station maintains traditional links with only a few UEs in the cluster, this is equivalent to that all UEs in the cluster maintains traditional links with the base station. One DUE in the DUE cluster is set as a cluster head, the cluster head maintains a traditional link with the base station, and is responsible for forwarding information from the base station. A node except the cluster head in the DUE cluster is referred to as a slave DUE. The slave DUE does not have a connection link with the cellular base station, or performs limited communication with the cellular base station, for example, the slave DUE is in a LTE-DETACHED state. Compared with a case that all UEs maintain the link connection with the base station, an interaction signaling is decreased greatly in a case that the slave DUE is not connected to the base station or performs the limited communication with the base station. The communication mode is not only suitable for a future communication scenario of high speed and dense cell deployment, but also suitable for a scenario of the field of public safety. However, a specific function of the cluster head and an operation required in the D2D communication and a corresponding operation of the slave DUE needs to be further planed and designed.